The invention concerns a check paper, that is forgery- and tamper proof, which is provided with crushable micro capsules, which contain a leuco ink, and with a color acceptor which reacts with the leuco ink to color development- or change. Furthermore, the invention concerns a method for the production of such a check paper.
Commercial dealings handle assets and claims to such assets with documents written or printed on paper. For example, such documents can be bank notes, checks or stocks. In these, as with all other cases where printed or written documents take on an importance, (documents, passports, travel- or airline tickets of high value) one has to make sure that the content of such documents cannot be altered at all or only with great difficulty. This is generally obtained by making it so, that manipulations on such documents leave visible traces which show the attempt to alter. For this purpose, one uses also reagents which are added to the paper. Such reagents respond very well to many tampering attempts with chemical means. However, frequently a protection against alteration attempts through mechanical erasing, is lacking.
It is known how to protect valuable prints against mechanical erasing attempts by means of complicated background design, whereby these background designs are formed in such a way that they are irreversibly changed or destroyed by erasing attempts. Thus, manipulation attempts, are easy to recognize. However, such security measures,-especially in the case of automatically processed valuable prints as checks and such, which are areas in which manipulations are usually tried, (for instance OCR-line)- can only be used to a limited extent since in many cases such background designs destroy the readability of the data that has to be automatically processed.
One can also secure paper against mechanical erasing by combining it from three differently colored layers. If one erases the whiter, outer layer of such a paper the result would be that the color of the layer below turns clearer which indicates an erasion attempt. However, such paper has to be manufactured on machines especially made for this and with a relatively high effort. The fact that the coloring of the layers and the resulting opaqueness makes it very difficult to apply a watermark it might even be useless if also considered a disadvantage.
It is also known to protect a paper against erasing by producing it with the least possible density. Thus, such a paper consists of a rather loose fiber bond where erasion attempts create a large destruction. This way the erasion attempt is visible, however, such a loosely processed paper has generally not strength factors required for most application purposes.
Another possibility for the protection of security papers against mechanical erasure is specified in the German patent application No. 16 596.
According to it, a check paper for checks and similar uses is already known, it consists of two glued together paper layers and has a color print at the inner surfaces. Mechanical erasion attempts wears on the corresponding paper layer and makes it thinner which emphasizes the lines of the inner print and shows the manipulation. Since the inner print is applied with sensitive aniline colors which run when erasing liquids are applied the check paper is thus also protected against chemical erasing. However, paper according to this invention does not meet the requirements needed for check paper nowadays. Glued together paper can be split in its layers and is therefore easily falsified and tampered with.
The use of micro capsules for the protection of security papers and passports has been specified in the German laid-open application No. 23 23 076: For this, the micro capsules have to be filled with liquid crystals which show a color change at a temperature compatible to the security paper. Hereby, these capsules have only the function of a validity characteristic, their aid shall help differentiate between falsification and the real security papers. For this, the check paper is subjected to a corresponding temperature whereby the usually invisible micro capsules become visible through their color change.
In this test, the micro capsules are not crushed, their casings can therefore be very stable. The known check paper which was treated with the micro capsules protects however in no way against mechanical erasion attempts and even as validity characteristic these micro capsules are only of a slight value, because the color change compared to the common requirements is too slow and the obtainable color contrasts are not sufficient.
It is not new to protect a security paper or check paper by applying on its surface micro-enclosed leuco inks as it is known from the technology of the copying papers. The application takes place either holohedral by staining or with a suitable printing machine with very low application pressure. If one erases at the thus protected locations, the mechanical pressure destroys the structure of the micro capsule and the color emerges. The reaction with certain color acceptors results in a visible coloring which proves the erasion attempt.
Securing check paper in this manner is specified for instance in the German patent application No. 26 000 781. Accordingly, entrance tickets, airline tickets or bus tickets are provided with a coating which consists of a spray varnish and imbedded micro capsules with leuco ink. This coating covers only one part of the surface of the check paper and has in addition the shape of a design which is preferably made of stripes or wave lines. The design is located at a point which is intended for the stamping or the writing on the check paper and does not cover it completely. During the labeling, and according to the design used, the micro capsules effected by the writing are destroyed, the outflowing leuco ink react with the color acceptor present in the paper structure and becomes visible because of the subsequent color reaction. A subsequent change of the imprinted information is only possible if the layer is completely worn down.
The known paper does not meet all requirements which are needed for a check paper which is protected against mechanical erasing. The micro capsules are exclusively on the surface of the paper and thus almost in the same plane as the printed information. Therefore, falsifications are not made difficult enough. Because, if one erases only a sufficiently thick layer of the surface one removes simultaneously the information and the information protecting, through the micro capsules caused image of this formation in the surface coating, still the paper remains at this point thick enough in order to apply subsequently a new falsified information. If one should succeed with chemical or mechanical means to dissolve the surface coating one removes simultaneously with it the information printed above without affecting the actual paper mass. Since the man skilled in the art is familiar with the technology and application of micro capsules with leuco inks, subsequently a new coating can be applied and the falsified information can be written on it. For the same reason, this known securing does not represent a protection against total falsifications. Another disadvantage has also to do with the arrangement of the micro capsules on the surface of the paper. Herewith, the coating lies not only in the area of the largest mechanical stress but it is also subjected to influences like surface damage, scratches etc. All these effects cause unintended discolorings of the check paper through the stresses it is subjected to.
The purpose of the invention is the production of a paper that is effectively protected by micro capsules against erasing. Hereby the presence of the micro capsules shall guarantee at the same time the validity of the check paper, that is, exclude total falsifications and enable the application of additional authenticity characteristics like the water mark for example. Furthermore, the check paper shall be made in a way that the stress occurring during circulation does not unintentionally discolor it.
According to invention this objective is met by the fact that the micro capsules and the color acceptor are present within the paper mass.
Advantageous further developments of the check paper according to invention are subject of the sub claims.
The manufacturing of the check paper according to invention takes place in the known prior art on a paper machine, however before the development of the sheet one adds micro enclosed leuco inks as well as reaction partners, that is, color acceptors to the paper material. If necessary the color acceptor can also be present in micro enclosed form. The condition for this manufacturing procedure is the coordination of wall thickness and strength of the micro capsules with the production devices which mechanically stress the paper so that during the production of the paper (including the various pressing- and drying processes) the micro capsules are not destroyed.
According to a preferred method, the paper is produced on a double-filter machine in such a way that the micro enclosed leuco ink is embedded in a web taken by the first sieve while the color acceptor is located in an additional web taken by a second sieve. These webs are joined and couched together before drying.
The check paper, produced according to the mentioned preferred manufacturing procedure has special advantages. By writing on the paper, the micro capsules, which are located under the letters are destroyed, the color reaction takes place mainly at the boundaries which are provided by the second manufacturing process. Only in this area are both reaction partners- and of those at least one micro-enclosed- existent in close proximity. Therefore, writing causes an image of the applied information in an internal area of the document. Naturally, this cannot be erased without destroying the paper structure visibly and lastingly. Since the application of the micro capsules in the specified manner requires the whole operating effort as well as the special know-how of the check paper production, the color reaction in an internal area of the check paper, is a reliable characteristic of its authenticity, similar to the way this is proven by the security thread.
Furthermore, the embedding of the micro capsules into the paper represents at the same time a mechanical protection for the capsules. There are no reaction partners with which the ink can react during an unintentional damage to the capsules in the outer layers near the micro capsules which are naturally subjected to special stress. Even though the capsules, capable to reaction within the check paper are protected relatively well by the paper itself, the rather high pressure affecting them during the writing causes the positive destruction of the micro capsules. If one removes from a security paper according to invention the applied information through mechanical erasing the image in the inner layer remains visible and indicates clearly a possible intent to falsify. Check paper manufactured in the specified manner is imprintable without destroying thereby the micro capsules. The commonly used printing methods, with the exception of the steel intaglio printing method, are perfectly suitable.
The suitable shaping of the relative densities of both paper layers permits it that the color-reaction zone is located more on the front or the back of the sheet. If for example the reaction zone lies mainly on the back of the sheet then the forgery attempt on the front of the sheet will appear to have been successful. Only an inspection of the back of the sheet would reveal the forgery attempt. On the the other hand, placing the reaction layer on the front of the sheet each forgery attempt would be immediately visible.